Environmental endocrine disruptors (EEDs) are often consequences of human activity; however, the effects of EEDs are not limited to humans. A primary focus over the past approximately 30years has been on chemical EEDs, but the repercussions of non-chemical EEDs, such as artificial light at night (LAN), are of increasing interest. The sensitivity of the circadian system to light and the influence of circadian organization on overall physiology and behavior make the system a target for disruption with widespread effects. Indeed, there is increasing evidence for a role of LAN in human health, including disruption of circadian regulation and melatonin signaling, metabolic dysregulation, cancer risk, and disruption of other hormonally-driven systems. These effects are not limited to humans; domesticated animals as well as wildlife are also exposed to LAN, and at risk for disrupted circadian rhythms. Here, we review data that support the role of LAN as an endocrine disruptor in humans to be considered in treatments and lifestyle suggestions. We also present the effects of LAN in other animals, and discuss the potential for ecosystem-wide effects of artificial LAN. This can inform decisions in agricultural practices and urban lighting decisions to avoid unintended outcomes.

Purpose New light sources including light emitting diodes (LEDs) have elicited questions about retinal damage, including the blue-light hazard. Some organizations have recommended avoiding using LEDs with correlated color temperatures (CCTs) exceeding 3000 K, since they tend to produce greater short-wavelength energy. This paper provides quantitative comparisons among light sources and use cases as they affect blue-light hazard. Methods The spectral radiant power characteristics of incandescent, fluorescent, LED and daylight sources were evaluated in terms of blue-light hazard using standard procedures for phakic, aphakic and pseudophakic eyes. Results Under most use cases, LEDs do not exhibit greater risk for blue-light hazard than other sources (e.g., incandescent). Because they generally produce little-to-no ultraviolet energy, LEDs often present less risk to aphakic eyes. Conclusions LEDs present no special concerns for blue-light hazard over some other common sources in typical use cases because photophobic responses limit exposure to bright sources. Where photophobic responses might not occur (e.g., eye surgery patients or premature infants) or where individuals suppress these responses (e.g., stage actors), caution is necessary. Evidence remains inconsistent regarding the risk of human retinal damage from long-term exposures to light insufficient to reach acute blue-light hazard thresholds.

In our recent paper on how artificial light at night (ALAN) affects within-individual changes in physiology, we used a unique experimental setup of colored LED lights to show effects on nighttime activity levels and physiology in free-living great tits, Parus major (Ouyang et al., 2017). Raap et al's response, entitled: “Rigorous field experiments are essential to understand the genuine severity of light pollution and to identify possible solutions” lists issues with our analyses (Raap et al., 2017). Rather than go into a detailed response, we use this forum to address the major critiques by answering the bigger question of what types of rigorous field experiments are needed to evaluate ALAN's impact. This article is protected by copyright. All rights reserved.

Approximately 20% of workers in developed countries are involved in night work. Nevertheless, many studies have strongly suggested that night-work-induced chronic circadian misalignment increases the risk of a diverse range of health problems. Although a relation between night work and irregular menstrual cycles has been indicated epidemiologically, a direct causal link remains elusive. Here, we report that repetitive reversal of light-dark (LD) cycles triggers irregular estrous cycles in mice. The findings showed that the estrous cycle remained irregular for more than four weeks after the mice were returned to regular LD cycles. Importantly, the magnitude of the negative impact of reversed LD cycles on the estrous cycle, or more specifically the decreased number of normal estrous cycles during the observation period, was dependent on the difference in the frequency of LD reversal. Presently, no clear solution to prevent night-work-mediated menstrual abnormalities is available, and reducing night work in modern society is difficult. Our findings indicate that optimizing work schedules could significantly prevent menstrual problems without reducing total night-work time.

Periodic, well timed exposure to light is important for our health and wellbeing. Light, in particular in the blue part of the spectrum, is thought to affect alertness both indirectly, by modifying circadian rhythms, and directly, giving rise to acute effects. We performed a systematic review of empirical studies on direct, acute effects of light on alertness to evaluate the reliability of these effects and to assess to what extent they depend on other factors, such as time of day, exposure duration and sleep pressure. In total, we identified 74 studies in which either light intensity, spectral distribution, or both were manipulated, and the effects on behavioral measures of alertness were evaluated, either subjectively or measured in performance tasks. The results show that increasing the intensity or the color temperature of polychromatic white light in general has been found to increase subjective ratings of alertness, though a substantial proportion of these studies failed to find significant effects. There is little evidence in the literature that these subjective alerting effects of light also translate into improvements on performance measures of alertness. For monochromatic or narrowband light exposure, some studies have shown improvement in reaction time tasks with exposure to blue light, but generally this was not accompanied by changes in subjective alertness. Thus, the alerting effects of light are far less clear than often suggested. We suggest that in future studies more attention should be paid to other factors that may influence the effects of light, such as chronotype, circadian phase, homeostatic state and prior light history.